1. Field of the Invention
This invention relates to a high viscosity xanthan and processes for the preparation of the high viscosity xanthan. One preferred embodiment is directed to dilution of a xanthan fermentation broth (solution) to produce a high viscosity xanthan. In another preferred embodiment pasteurization of a xanthan fermentation broth (solution) by a direct steam injection system provides a xanthan having increased viscosity compared to its native form. This invention is further related to a high viscosity xanthan produced by the processes of this invention.
2. Related Background Art
The fermentation of carbohydrates to produce the biosynthetic water-soluble polysaccharide xanthan gum by the action of Xanthomonas bacteria is well known. The earliest work was conducted by the United States Department of Agriculture and is described in U.S. Pat. No. 3,000,790. Xanthomonas hydrophilic colloid (xe2x80x9cxanthanxe2x80x9d) is an exocellular heteropolysaccharide. The heteropolysaccharide has a backbone chain of (1xe2x86x924)-xcex2-glucose residues substituted by short, lateral chains linked to alternate monomeric residues of the main chain (Milas and Rinaudo, Carbohydrate Research, 76, 189-196, 1979). Xanthan has a wide variety of industrial applications including use in oil well drilling muds, as a viscosity control additive in secondary recovery of petroleum by water flooding, as a thickener in foods, as a stabilizing agent, and as a emulsifying, suspending and sizing agent (Encyclopedia of Polymer Science and Engineering, 2nd Edition, Editors John Wiley and Sons, 901-918, 1989). Xanthan can also be used in cosmetic preparations, pharmaceutical vehicles and similar compositions.
Xanthan is produced by aerobic submerged fermentation of a bacterium of the genus Xanthomonas. The fermentation medium contains carbohydrate (such as sugar), trace elements and other nutrients. Once fermentation is complete, the resulting fermentation broth (solution) is heat-treated. It is well established that heat treatment of xanthan fermentation broths and solutions leads to a conformational change of native xanthan at or above a transition temperature (xe2x80x9cTmxe2x80x9d) to produce a higher viscosity xanthan. Heat treatment also has the beneficial effect of destroying viable microorganisms and undesired enzyme activities in the xanthan. Following heat-treatment, the xanthan is recovered by alcohol precipitation. However, heat treatment of xanthan fermentation broths (solutions) also has disadvantages, such as thermal degradation of the xanthan. Heating xanthan solutions or broths beyond Tm or holding them at temperatures above Tm for more than a few seconds leads to thermal degradation of the xanthan. Degradation of xanthan irreversibly reduces its viscosity. Accordingly, heat treatment is an important technique with which to control the quality and consistency of xanthan.
Xanthan quality is primarily determined by two viscosity tests that are well known to those skilled in the art, i.e., the Low Shear Rate Viscosity (xe2x80x9cLSRVxe2x80x9d) in tap water solutions and the Sea Water Viscosity (xe2x80x9cSWVxe2x80x9d) in high salt solutions. Pasteurization of xanthan fermentation broths at temperatures at or above Tm have been found to result in the recovered xanthan having a higher viscosity as indicated by higher LSRV and SWV values.
The processes occurring during thermal treatment of xanthan fermentation broth (solution) are not well understood. It is believed that two processes occur during the thermal treatment of xanthan fermentation broth. Firstly, the thermal heating induces a conformation change which unwinds the double stranded native xanthan into a disordered xanthan which on cooling renatures to form an ordered xanthan. The renatured ordered xanthan has a higher viscosity compared to the native ordered xanthan which has not been pasteurized. Secondly, a competing degradation process may occur during the pasteurization (heat-treatment) which cleaves the native xanthan strands into shorter pieces resulting in a substantial decrease in viscosity. Studies have shown that the processes resulting in the desired conformational change occur rapidly, on the order of seconds, once Tm is reached (Norton et al., Journal of Molecular Biology, 175, 371-394, 1984). Conversely, the thermal degradation process is slow, on the order of minutes at typical operating temperatures, but accelerates with increasing temperature. Below a temperature of 70xc2x0 C., thermal degradation appears to be negligible. Maximum xanthan viscosity is achieved by rapidly heating the xanthan broth at or above Tm followed by rapid cooling. It is generally important to minimize the time that the fermentation broth temperature exceeds 70xc2x0 C. in order to preserve the integrity of the final xanthan.
The relative viscosity at 25xc2x0 C. of both native unpasteurized xanthan solutions and solutions which were previously pasteurized at various temperatures have been measured (Milas and Rinaudo, Polymer Bulletin, 12, 507-514, 1984). For solutions previously heated well above their Tm, a greater than three fold increase in the relative viscosity of the xanthan was observed. For solutions previously heated to temperatures near their Tm, partial increases in the relative viscosity, reflecting the fraction of molecules undergoing the conformational transition as determined from optical rotation measurements, were observed. These viscosity results, in addition to the observed hysteresis of the optical rotation curve upon an initial heating and cooling cycle of a xanthan solution, are consistent with the following model: 
A maximum viscosity will be obtained when all xanthan molecules have been transformed from the native to the renatured conformation. However, if Tm is high (xe2x89xa7100xc2x0 C.), pasteurization of the xanthan broth above Tm can result in degradation of the xanthan molecules and a resulting decrease in product viscosity (lower LSRV and SWV values).
One of the variables Tm depends upon is the ionic strength of the fermentation solution. The Tm of aqueous xanthan solutions as a function of added sodium chloride has been measured (Milas and Rinaudo, Carbohydrate Research, 76, 189-196, 1979). The Tm was found to vary linearly with the log of the total ionic strength (the ionic strength contains contributions from both the added sodium chloride and the xanthan molecules). The Tm of xanthan has also been monitored in aqueous potassium chloride solutions using differential scanning calorimetry (Norton et al., Journal of Molecular Biology, 175, 371-394, 1984). A linear dependence of Tm on the log of the ionic strength was also found.
A problem that has been identified with current recovery processes used to produce xanthan is the difficulty in achieving uniform heating of the broth. In particular, during pasteurization of the fermentation broth (solution) it is difficult to heat the broth in a uniform, controlled manner. As a result, some variability in the viscosity of xanthan recovered at the completion of the process may be observed. This may also give rise to reproducibility problems for xanthans having certain desired characteristics. Heating in a uniform and controlled manner is also dependent in part on the characteristics of the solution being heated. For example, xanthan fermentation broths are very viscous yet pseudoplastic (shear-thinning). When high concentration xanthan solutions are heated in conventional heat exchangers, non-uniform heating will occur. It is difficult to achieve a uniform temperature and residence time for every element of fluid passing through conventional heat exchange equipment during pasteurization of xanthan. Thus, during the heating of a xanthan fermentor broth, the conductive and convective heat transfer are poor and the formation of unwanted temperature gradients in the broth are difficult to prevent. This results in xanthan viscosities far below what is theoretically achievable. For example, this occurs in a xanthan solution passing through a heated tube. Because of the high viscosity of xanthan solutions, turbulent mixing (the primary mechanism of heat transfer in non-viscous liquids passing over a heated surface) in a heated tube is minimal and heat transfer occurs primarily by a much slower conduction mechanism. In addition, the layers of fluid near the wall of the tube experience a higher shearing force and are thus less viscous than the layers in the center. This causes the center core of fluid to behave as a plug and reduces mixing with the higher temperature outer layers. In order for heat transfer to occur, the surface temperature of the tube wall must be greater than the desired average temperature of the xanthan solution. With poor mixing of the fluid, the outer layers of fluid tend to be overheated while layers in the center are under-heated. When these heat exchangers are dismantled for cleaning, the inside walls of the heated tubes are heavily coated with burned xanthan.
The poor fluid mixing in conventional heat transfer equipment also significantly reduces the efficiency of heat transfer, necessitating the use of very large surface area (large volume) heat exchangers. The use of large volume heat exchangers cause the xanthan solutions to remain at elevated temperatures long enough for thermal degradation of the polymer to begin, thus reducing the maximum viscosity achievable. In addition, the large mass of fluid and metal present in this type of equipment leads to poor automated process control because there is a time lag between changes in the input of the heating medium (usually steam) and the temperature response in the xanthan solution being heated. The result is a varying output temperature in the xanthan solution and sluggish response to temperature changes even when computer control is used.
Conventional production sized heat exchangers also suffer from another problem when processing highly pseudoplastic fluids like xanthan. To increase the surface area available for heat transfer in a heat exchanger, many heated tubes or slots are placed in parallel with each other so that the fluid is divided into many smaller streams. The alternative would be to use a single channel of impracticably long length. Each channel will have a slightly different cross sectional area, leading to differing amounts of shear in each channel. Because xanthan is highly shear thinning, the result is uneven amounts of flow in different channels, leading again to uneven heating.
The problems described above lead to non-uniform heat treatment of the xanthan solution. This results in less than optimum development of xanthan viscosity, thermal degradation of the xanthan, incomplete destruction of unwanted enzymes and poor process control. A more effective process of heating xanthan solutions at or above Tm for viscosity enhancement that will reduce loss of xanthan viscosity due to degradation from overheating of the xanthan solution or broth would be highly desirable. Furthermore, it would also be highly desirable to provide for processes that enhance the viscosity characteristics of xanthan while overcoming the current limitations encountered in the preparation of xanthan from fermentation broths (solutions). Xanthans that have been prepared by dilution of the fermentation broth with 25 percent or less of a diluent have been commercially available. Such xanthans have been pasteurized by steam injection have also been available. However, xanthans having improved viscosities over xanthans prepared in the above-described manner would be highly desirable. This invention provides a number of effective processes for enhancing the viscosity of xanthan, in addition to providing a process in which xanthan can be heated in a more uniform manner while attaining the desired characteristic of high viscosity.
This invention relates to a high viscosity xanthan and processes for producing said high viscosity xanthan compared to its unpasteurized native xanthan form.
One aspect of this invention is related to a process for increasing the viscosity of xanthan comprising the steps of: (i) diluting xanthan in a fermentation broth with more than 30 percent by volume of a diluent to said fermentation broth volume; and (ii) pasteurizing said diluted xanthan fermentation broth. A particularly preferred pasteurization process is to utilize steam from a direct steam injection system.
More specifically, the invention relates to a process for increasing viscosity of a xanthan comprising the following steps:
(i) producing said xanthan in a fermentation broth by aerobic fermentation of a bacterium:
(ii) diluting said xanthan in said fermentation broth with more than 30 percent by volume of a diluent to said fermentation broth volume;
(iii) pasteurizing said diluted xanthan using steam from a direct steam injection system;
(iv) cooling said pasteurized fermentation broth to an ambient temperature; and
(v) precipitating said xanthan having an increased viscosity.
Yet another embodiment of this invention is directed to high viscosity xanthan prepared by the above-described processes. Xanthans with increased viscosities have improved product functionality for a number of different commercial applications are produced using the processes of the present invention.